Annalee Newitz

As we move into a clean tech future, it's becoming more obvious that the old distinction between machines and nature is a false one. This sophisticated filter, made by researchers at MIT, is a perfect way to remove bacteria from drinking water — and its main component is xylem, or the connective tissues of a tree.

Water and nutrients absorbed by trees must first pass through the tree's xylem, a porous tissue that you can find at the heart of a tree branch. Scientists had speculated that this xylem was a filtration system that would remove toxins and other unwanted particles, but they weren't sure how well it worked.

So Rohit Karnik and his colleagues designed a very simple test. They plugged a tube with some xylem, used it to filter some water, and examined the results. Not only did the xylem filter out common bacteria like E. coli (which is responsible for a lot of food poisoning), but it also filtered out any other particle whose diameter was bigger than 100 nanometers. It even filtered out some red pigment they added to the water.

To find out exactly how the xylem works, they cut open the filters and studied the internal structure of the wood. They found that most of the filtering occurs in the first two or 3 millimetres of the filter. That turns out to be more or less exactly the length of the tracheid cells and suggests that the wood filters can be cut even shorter and still function effectively.

They also took electron microscope images of the cell pits which showed that the bacteria collected near these pits, suggesting that this is indeed the mechanism of filtration.

There are also a lot of limits to the technology. Viruses are smaller than 100 nanometers, so the xylem filter won't work against them. And the xylem also needs to be fresh to be effective. The researchers suggest that different types of xylem might be effective against viruses and other small contaminants. They also hope to explore drying techniques that would allow people to use xylem filters even when they aren't freshly cut.

More work needs to be done, but these intrepid researchers have clearly demonstrated that natural materials can function as parts of a machine. Get ready for a future where you grow your waters in the backyard.